This invention relates in general to optical apparatus and in particular to a new and useful optical elements comprising a system of a plurality of infrared transmitting optically effective thin layers applied to an infrared transmitting substrate and an intermediate layer of a thickness of 100 mm at most therebetween.
Optical elements are used, for example, in the form of infrared transmitting lenses having a reflection reducing coating, as narrow-band transmitting filters, and as polarization filters for infrared. Frequently, in such application, the adhesion of the layer systems to the substrates and of the layers to each other still poses a problem in infrared layer systems particularly due to the fact that both the infrared transmitting substrates and the layer materials are somewhat sensitive to air humidity and oxidation and the prior art infrared transmitting layer materials frequently show high layer stresses.
It is known to improve the adhesion of layers to substrates by so called adhesion layers which are provided between the substrate and the system of layers and are so thin that they in themselves do not disturb the function of the optical element. In optical elements intended for the visible spectral region, adhesion layers primarily of oxides, have already been employed in many instances.
The situation is more difficult with optical elements for the IR region, particularly in some recent application where layer systems comprising a great number, up to 50, of individual layers are applied to IR transmitting substrates. The reason for this difficulty is that the materials in consideration as adhesion layers are mostly relatively strongly absorbing, particularly after they have taken up moisture and, consequently, exhibit H.sub.2 O absorption bands. Quite particularly important are absorption losses in laser mirrors which, with a maximum reflectance, must have a minimum absorption, to be capable of withstanding high initial intensities of the laser beam.
With IR layer systems to be applied to a germanium substrate it is known to obtain a better adhesion between the substrate and the layer system by interposing an adhesion layer of germanium which need only be some few atom layers thick. Further known are protective layers for optical elements, which are deposited on the surface to be protected by means of an electric glow discharge from the vapor of organic silicon components.